In conventional natural gas conditioning, natural gas, having passed through an acid gas removal unit (AGRU) and dewpoint control, is often dehydrated by passing the natural gas through a system of vessels or units referred to as a dehydration unit containing adsorption beds made up of particulate material, also referred to interchangeably herein as solid desiccant, molecular sieve or mole sieve. Such a system includes at least two vessels in which one of the vessels contains saturated mole sieve that is in regeneration mode, while the other one or more vessels are operated in dehydration mode. During dehydration mode, water and other contaminants are adsorbed onto the mole sieve material; and during regeneration mode, they are desorbed from the mole sieve. Typically, the regeneration begins by passing hot dry natural gas, i.e., natural gas having been dehydrated, over the saturated mole sieve. This requires a large compressor to return hot dry natural gas to a location upstream of the dehydration unit or the AGRU.
Dehydration of natural gas is typically accomplished by flow of hot gas over zeolite-based molecular sieve adsorbent. Water in the gas is preferentially adsorbed by the molecular sieve. Removal of water from the gas using molecular sieve dehydration is a vital process component in any liquefied natural gas (LNG) plant to meet moisture content specifications (down to 0.1 ppmv). Natural gas can contain additional contaminants such as hydrogen sulfide, mercaptans, oxygen, carbon dioxide, carbonyl sulfide, etc. that are partially co-adsorbed by the molecular sieve. During high pressure regeneration, system design problems such as hydrocarbon and water refluxing can result in poor water desorption (high residual water content within the mole sieve) and corrosion. This can result in early moisture breakthrough and economic losses associated with frequent mole sieve change-outs and low dehydrator availability.
If the mole sieve bed is regenerated at high temperature and low pressure, then the regeneration gas may be a slip stream of dry gas, LNG boil off gas, or any other suitable dry gases. If the regeneration is conducted at high pressure and large vessel diameters, then the vessel thickness and choice of materials will create additional heat load on the regeneration system.
The regeneration gas contains contaminants such as oxygen that reacts with hydrogen, hydrogen sulfide and/or hydrocarbon (e.g. propane) at high regeneration temperatures resulting in the formation of unwanted by-products such as sulfur, sulfur di-oxides, water and carbon dioxide. These by-products can build up in downstream units, or in the fuel system causing problems such as fouling, and off-specification products. Furthermore, the complete regeneration of molecular sieves is not achieved because of the contaminants present resulting in sub-optimal performance of the dehydration unit. This may also be accompanied by damage caused to the molecular sieve resulting in reduced operating life. One known solution is further purification of the regeneration gas by using additional adsorbents. However, such schemes are expensive and will not always result in full contaminant removal of the regeneration gas.
There exists a need for a more efficient, more reliable and less costly method and system for regenerating saturated mole sieve in a natural gas dehydration unit.